Described herein is a method and system for locating a current position and a coupling location of a mobile unit by way of a leaky waveguide.
In communication technology, there is a problem of reliable communication and moreover the problem of locating transmitters borne by persons or vehicles in tunnels, such as in the transport of personnel, in the construction of tunnels or in underground mining.
In this field, special devices are used for sending and receiving, such as slit or leaky waveguides as cables, are used for communication with mobile phones, for example. Such leaky waveguides are in effect coaxial cables provided with slits in the shielding area. Depending on the propagation direction, an electromagnetic wave is coupled into the air or into the cable through these slits. Due to the long extension of the cable and the coupling mechanism, this is a complex process in the near field of the antenna, i.e. the transition cannot be easily simulated and is not to be approximated by the usual remote-field approximation processes.
This is why the well-known attempts to locate radio transmitters by way of determining their field strengths or their field strength decay, remain extremely imprecise. Such an attempt is known, for example, from [Weber] Weber et al., “Indoor RF Fingerprinting using leaky feeder cable considering environmental changes”, Proceedings of ACM Mobility Conference 2009, Sep. 2-4, 2009, Nice, France. Moreover, the essential portion of the power decay and the power fluctuation occurs over the distance between the radio unit and the cable, in an amount of 30-70 dB for example, and the power decay along the cable is very small, at 1.5 dB/m, for example. This is why even small variations in field strength substantially falsify the result.
Apart from measuring the field strength, a propagation delay can also be measured. It is, however, technically complex to measure it along the leaky waveguide. If, for example, the distance to one side is measured by way of round-trip measurement (round-trip time-of-flight), it will include both the propagation delay through the air and the propagation delay along the cable. Measuring the distance to one end and then to the other in a sequential manner is principally possible, it needs precise tuning of the measuring time slots, however, doubles communication overhead and is exceedingly error prone due to the sequential measurement and the rapidly varying radio channel. A further disadvantage is the necessity of two-way communication, because it is technically more complex, and also the maximum possible measuring rate is reduced as the number of mobile units increases.
Nishikawa et al, “A new position detection method using Leaky Coaxial Cable”, IEICE Electronics Express, vol. 5, no. 8, pp. 285-290, 2008, describes, for example, an arrangement and a method for determining the position of a receiver, in which a leaky waveguide is used. The leaky waveguide comprises sequential groups of slits, wherein the slits of the one group are inclined in one direction and the slits of the other group are inclined in the opposite direction. The receiver directly receives a signal fed into one end of the leaky waveguide and at a later time the same signal after it has been reflected at the other end of the leaky waveguide. The difference in time between the directly received signal and the reflected received signal is determined by the receiver. The method used cannot be implemented in practice, however, due to the high dynamic differences and the interference of the communication by the termination, in particular so-called intersymbol interference.
Such a configuration by way of a faulty termination of the second cable end and the determination of the distance from the difference of the two echoes involves substantial technical drawbacks. Due to the great dynamic difference of the two signals, they cannot be safely distinguished. Moreover, the signals overlap each other during location near the unadapted cable end and cannot be resolved, which means that position determination is not possible there. A further drawback is that when the cable is simultaneously used for communication, it is substantially interfered with due to reflections at the loose end, such as by the occurrence of intersymbol interference (ISI). The above mentioned drawbacks of the necessary two-way communication, such as measuring rate, technical complexity, also remain valid.
In a different technical field, such as the location of vehicles in large hangars and in logistic centers, a method for synchronizing clock devices is known, from U.S. Pat. No. 7,594,133 B2 [Sym06], in which a transmitting unit sends at least one narrow-band pre-signal, clock devices of receiving units are pre-synchronized by receiving units by coupling onto such pre-signal of the source of the pre-signal, the transmitting unit sends a broad-band measuring signal after a certain waiting period and the receiving units receive the signal, the receiving units correlate the broad-band measuring signal with a comparison signal modulated in the same way and, based on the correlation result, the receiving time of the broad-band measuring signal is determined and the deviation of the synchronization of the clock devices is determined and compensated. Such existing radio-location determining methods must be realized with high technical overhead for the infrastructure, such as a great number of stationary reference units, due to propagation conditions, such as reflection, shading and propagation delay spread, and are thus not widely used outside of the field of large hangars and logistic centers, for example. Location determining transmitters borne by persons and vehicles in tunnels, for example in the construction of tunnels or in underground mining is thus also a problem which is technically challenging.
In this technical field, which is extraneous to the field of communication technology, a method is known from German Patent No. DE 101 57 931 C2, or U.S. Pat. No. 7,940,743 B2 [Sym16] for synchronizing radio stations with respect to each other and a time-synchronous bus system. A transmitter station and a receiver station communicate through an interface, wherein a transmitting signal is generated in a transmitter station by way of a signal source, and sent through the interface, and wherein a corresponding receiving signal is received by the interface in the receiver station. To synchronize the transmitter station and receiver station, a receiver signal-source signal is evaluated by a receiver-side signal source tuned to the sender-side signal source. The same frequency modulation is applied to both the transmitting signal and the receiver signal-source signal. The receiving signal received in the receiver station is mixed with the receiver signal-source signal to create a mixed signal, and the mixed signal is spectrally analyzed. Again, this method has been developed, in particular, for the field of location determining vehicles and goods in large hangars and in logistic centers.